Field of the Invention
The present invention relates to a positive electrode active material with improved output and a lithium secondary battery comprising the same, and more particularly, to a mixed positive electrode active material relaxing a rapid output decrease in a low SOC section to widen an available SOC section by blending Mn-rich and Co-doped Li4Mn5O12, and providing further improved output when compared with a case using pure Li4Mn5O12, and a lithium secondary battery comprising the same.
Description of the Related Art
Recently, lithium secondary batteries are used in many fields including portable electronic devices such as cellular phones, PDAs, laptop computers, and the like. Particularly, according to the increase on environmental problems, researches on the lithium secondary batteries having a high energy density and a high discharge voltage are actively conducted as a driving source of electric vehicles capable of replacing vehicles using fossil fuel which is one of the main factors on atmospheric contamination, such as gasoline vehicles and diesel vehicles, and some of the batteries are at a commercialization step. In order to use the lithium secondary batteries as the driving source in the electric vehicles, a high output and a stable output in a state of charge (SOC) is required to be maintained.
Electric vehicles are classified into a typical electric vehicle (EV), a battery electric vehicle (BEV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV) and the like.
The HEV is a vehicle obtaining a driving force from the combination of a common internal-combustion (engine) and an electric battery. The driving is mainly accomplished through the engine, and the battery assists the deficient output of the engine when a greater output is required than a common case as driving an uphill road, etc. During the stop of the vehicle, the battery is charged to recover the SOC again. That is, the main driving source for the HEV is the engine, and the battery is an assisting driving source and is intermittently used.
The PHEV is a vehicle obtaining a driving force from the combination of an engine and a rechargeable battery connected to an external power supply, and is mainly classified into a parallel type PHEV and a series type PHEV.
In the parallel type PHEV, the engine and the battery have an equal relation as the driving sources. The engine and the battery operate alternately as the main driving source according to the circumstances. When the engine is the main driving source, the battery assists the deficient output of the engine, and when the battery is the main driving source, the engine assists the deficient output of the battery. Thus, the engine and the battery work in parallel.
However, the series type PHEV is basically driven by the battery only, and the engine only performs the charging of the battery. Different from the HEV or the parallel type PHEV, the series type PHEV entirely depends on the battery instead of the engine for the driving of the vehicles. Thus, in order to achieve a vehicle running stability, the maintenance of a stable output according to the battery properties in the using SOC section is a relatively significant factor when comparing with other kinds of the electric vehicles, likewise for the EV.
Meanwhile, as the positive electrode material of a high capacity lithium secondary battery, a commonly used representative positive electrode material, LiCoO2 has a practical limitation on the increase of the energy density and the output properties. Particularly when LiCoO2 is used in high energy density fields, structural deformation at a high temperature charge state due to a structural instability may be induced, and oxygen may be generated to make an exothermic reaction with an electrolyte in the battery to cause a battery explosion. In order to improve the safety of LiCoO2, the use of lithium containing manganese oxides such as LiMnO2 having a lamella crystalline structure, LiMn2O4 having a spinel crystalline structure, etc. and lithium-containing nickel oxide (LiNiO2) has been considered. Recently, researches on lithium manganese oxides obtained by adding a relatively large amount of Mn as an essential transition metal than other transition metals (excluding lithium) into lithium manganese oxides having a lamellar structure (hereinafter, will be referred to as “Mn-rich”), are conducted a lot as a high capacity material.
The Mn-rich has a high output in a high SOC section (for example, the SOC section of 50 or over), however, has a rapidly decreasing output at a low SOC section due to the increase of resistance. Thus, the positive electrode material of the lithium secondary battery used in the series type PHEV or EV has a limitation in application. The defects may be illustrated even when mixing a positive electrode active material having a higher operating voltage than the Mn-rich because only the Mn-rich operates in the low SOC section.
The above-described point is a main obstacle in applying the high capacity Mn-rich in fields requiring good output properties such as electric vehicles. Particularly, different from an HEV using an engine as a main operating source and a parallel type PHEV using an engine and a battery as equal operating sources, the series type PHEV or the EV using only a battery in operating vehicles may be used only in an SOC section maintaining a required output or over. When the Mn-rich is used alone as the positive electrode active material, the output in a low SOC section may be deteriorated, and an available SOC section may be largely decreased.
In order to solve the above-described defects, a method of blending a material having an operating voltage of about 3 V, for example, Li4Mn5O12, with the Mn-rich may be considered. However, since Li4Mn5O12 has a large polarization loss and relatively inferior rate properties, a satisfactory output increase with the Mn-rich may not be expected even though performing the blending with the Mn-rich.
Therefore, developments on Mn-rich positive electrode materials maintaining an output in a low SOC section to enlarge an available SOC section and exhibit a high output required in electric vehicles and medium-large size devices are urgent.